1. Field of the Invention
This invention relates to an image forming apparatus, such as a copying machine, a printer, a facsimile machine, and more particularly, to an apparatus having a combination of belt driving units, in which respective belts are disposed in contact with and tensioned around a plurality of rollers such that the belts are rotated at the same circumferential speed in contact with each other, so as to transfer a toner image from one belt to the other.
2. Discussion of the Related Art
An image forming apparatus such as a copying machine is known to include, for example, an image bearing member for forming an electrostatic latent image, a developing member for developing the electrostatic latent image with toner to form a toner image, a first transfer unit for transferring the toner image thereto, and a second transfer unit. The second transfer unit transfers the toner image from the first transfer unit to a transfer sheet of paper which is fed by the second transfer unit into a second transfer region disposed between the first transfer unit and the second transfer unit.
The first transfer unit and the second transfer unit, which in such an image forming apparatus, include a combination of units arranged to transfer a toner image. The first and second transfer units move in contact with an adjacent mobile unit with the same circumferential speed, and in many cases, each unit includes an endless belt which is spanned and tensioned around a plurality of rollers. In this case, the second transfer unit also operates as a feeding unit for a transfer sheet of paper.
Such a transfer unit having a belt is advantageous in designing the apparatus, because it provides a degree of freedom in designing the configuration of the apparatus. Therefore, a combination of the transfer units having belts, for example, a combination of an image bearing member and a first transfer unit or a combination of a first transfer unit and a second transfer unit, is frequently included and housed in an image forming apparatus.
In such an image forming apparatus having a combination of transfer units having belts as described above, a toner image of the first transfer unit is transferred to the second transfer unit or to a transfer sheet of paper using an electrostatic force in a transfer region, which defines a nip between the belts at which the belts are brought into tight contact each other.
A driving roller, which is one of the plurality of rollers that drives one of the first and second belts, controls rotation of the units having the belt. In this case, a sufficiently high tension is imposed on the belt that is spanned around the plurality of rollers so that the driving force is reliably transmitted from the driving roller to the belt.
During a period of time when the unit including the belt is in a stopped state, the transfer unit is at rest, which causes high-tension to be applied to the above-mentioned belt. If such a high-tension state is applied to the belt for a long period of time, portions of the belt, which are disposed along respective circumferential surfaces of the rollers, may be deformed via plastic deformation. Such plastic deformation is referred to as "the curl tendency". Hereinafter, a portion which is deformed by plastic deformation due to extended contact with the roller is referred to as "a deformed portion."
In the image forming apparatus described above, which includes a combination of transfer units having respective belts which are deformed by plastic deformation, when a deformed portion of the first belt overlaps another deformed portion of the second belt in a transfer region, the capability of transferring the image is deteriorated. This is caused by the inability to maintain tight contact between the belts, which frequently occurs in the transfer region due to the plastic deformation. In particular, the longer the time that the belts and rollers are stopped and at rest, the greater the degree of plastic deformation becomes, which further decreases the tight contact between the belts. Accordingly, the image quality is often deteriorated. For example, when an undesirable micro gap between the combination of belts is generated in the transfer region, occurrence of an unusual discharge through the gap causes greatly decreased image quality.